Lessons Learned in Laying Out the Piscataway WRRF Bio-Energy Project

The Washington Suburban Sanitary Commission (WSSC) is in the process of implementing a Bio-Energy project to thermally hydrolyze and anaerobically digest all biosolids from their five Water Resource Recovery Facilities (WRRFs) to produce a Class A biosolids product. This presentation will discuss four key design considerations and lessons learned in laying out this facility: - Increasing reliability and availability of the Thermal Hydrolysis Plant (THP), - Design considerations for cake and sludge piping around the THP, - Challenges of an evolving market for combined head and Power (CHP) boilers on a THP project, - Squeezing the value out of biosolids. Consideration 1 Characteristics of the THP process make it potentially damaging to equipment. The inherent risks of piping and equipment failure create a need for periodic preventive and corrective maintenance shutdown for inspections and repairs, which will lead to annual periods of unavailability of the THP. Unavailability of the THP creates logistic issues with biosolids management and potential upsets downstream biological processes (digesters, sidestream), both of which the design must consider. Solution 1 A standard way to increase availability and reliability of a process is to provide this process with redundant units. When this option became available, WSSC chose to implement it through installation of add-on modules to the THP system and minor piping modifications. Consideration 2 Cake and sludge conveyance is energy intensive, and energy use is proportional to the length of the pipe. There are limits on pumps and pipe fittings that determine the maximum pressures a system can be designed for. In an ideal situation, cake reception (if using imported cake), storage, THP and digesters should be co-located, but when retrofitting an existing facility, this becomes a mighty challenge. Dual phase piping from the THP to the digesters requires a continuously ascending slope. Flow interruption on hydrolyzed sludge can create a big sticky mess. Solution 2 Most common methods to limit the headloss include use of long turning radius elbows, use of boundary layer injection (water or polymer), and other methods have been introduced in the market, such as air assisted cake pumping. When laying out a THP project, giving these elements the proper priority and importance can save big capital and operation costs. Consideration 3 Boilers in CHP systems for THP projects often have the ability to use engine generator exhaust gas and biogas along with other fuels (normally natural gas), for reliability and optimization of the energy use. They additionally need to be responsive to the steam demands which are inherently variable because THP is a semi-batch process. In Europe, composite boilers are the standard for THP projects, but the US market has not yet evolved to a point where these units are available. Instead, the standard approach for stream production from engines for CHP units is the use of heat recovery steam generators (HRSG). Bringing standard composite boilers to the US is not only extremely expensive, but also a total nightmare from the point of view of code compliance. The biogas boilers market in the US has mostly evolved from the landfill industry and there are limited options when it comes to using both engine exhaust gases and biogas. Solution 3 Available options in the US use co-firing of exhaust gas and biogas and provide limited windows of operation when it comes to limiting flame-out of the burners, due to the presence of the CO2 that comes along with the biogas. Innovative approaches using gas recirculation and multi-variable controls provide a feasible solution. Consideration 4 There are many paths to squeeze value out of the biosolids, which many times are focused on the use of THP to increase volatile solids destruction in the downstream anaerobic digestion, producing a higher value Class A biosolids, reducing hauling costs and increasing biogas production, but there are additional approaches to capture more value from THP process. Some examples of additional value include the use of biogas to generate power on engine generators, which can be coupled with a boiler than can use engine exhaust gases to improve the CHP unit overall efficiency. At Piscataway we set out to install CHP units that would serve the dual purpose of providing steam to the THP and electricity for the plant, as well as improving the energy independence from the grid. The WSSC bioenergy project is aimed at capturing the maximum value of the process without compromising the very real capital investment constraints. Solution 4 To achieve this optimization, we set out to design a CHP unit that would allow the project to apply for an energy grant offered by the State of Maryland, to CHP units that would achieve a minimum 60% energy efficiency. To reach this challenging goal, the CHP unit was designed in a way that not only included electrically efficient engines, and a boiler that uses the engine exhaust gases, but the overall system efficiency was supplemented with energy recovery from the engine secondary cooling system, through integration of this system with the Sidestream treatment process and pre-heating of the boiler feed water. Later in the project, a new more challenging energy efficiency goal was set, to achieve a 65% efficiency, to access additional grant funds from the local electrical utility company. Multiple options where analyzed, some of which were immediately discarded, some of which were later tossed out due to the complexity of their implementation and/or cost, and two were selected for implementation. The first one associated with additional energy recovery via additional preheating of the boiler feed water on economizers, and the second one, uses the heat from the engine cooling system to preheat the dilution water used to adjust the solids content in the THP feed.